Interpretation ID: nht74-2.18

DATE: 06/07/74

FROM: AUTHOR UNAVAILABLE; R. B. Dyson; NHTSA

TO: Certain-Teed Products Corporation

TITLE: FMVSS INTERPRETATION

TEXT: This responds to your April 5, 1974, question whether S5.6.1 or S5.6.2 of Standard No. 121, Air brake systems, requires parking brakes on all axles other than front steerable axles.

S5.6.1 requires "parking brakes on an axle other than a steerable front axle" to have certain static retardation force values. "An axle" refers to any axle other than steerable front axles and therefore S5.6.1 requires parking brakes on all axles other than steerable front axles. A tandem axle consists of two separate axles for purposes of this requirement.

S5.6.2 has no specific axle-by-axle requirements. Its performance standard may be met by any means which hold the vehicle stationary and conform to S5.6.3 and S5.6.4. It should be emphasized that this requirement cannot be met simply be equipping the vehicle with parking brakes which hold to the limit of tractive ability but permit vehicle movement.

Yours truly,

ATTACH.

CERTAIN-TEED PRODUCTS CORPORATION

April 5, 1974

Chief Council -- National Highway Traffic Safety Adm., U.S. Department of Transportation

RE: Federal Vehicle Motor Safety Standards No. 121-Air Brake System Docket No. 74-10; Notice 1

Dear Sir:

We are requesting clearification of Paragraph S. 5.6.1 and S. 5.6.2.

Is the intent of Paragraph S. 5.6.1 to require parking brakes on all axles other than a steerable front axle, and is a tandum axle from the viewpoint of this regulation considered to be made up of two (2) separate axles?

In Paragraph S. 5.6.2, is it necessary to have parking brakes on each axle of the vehicle, or would a vehicle be acceptable if parking brakes mounted on only a portion of the axle were adequate to meet the grade holding requirements.

There appears to be questions in the minds of vehicle manufacturers, who are our customers, as to whether a trailer or a truck and bus with a modulated emergency system would require parking brakes on all non-steerable axles or whether only a portion of the axle requires parking brakes. We would appreciate an early reply to this inquiry.

Yours very truly,

B. D. Sibley -- Chief Product Engineer, Brake Products